In most countries, there isn't a dress code at the airport. You can wear what you want so long as it's compliant with whatever security need from you. That means a onesie probably isn't breaking the rules - but you're probably inviting the security team to stop and search you.
What even is the onesie, anyway? Is he supposed to be an alligator? A crocodile? Some kind of dinosaur? We can't work it out! At least he seems to be happy with it.